This invention relates generally to an N-channel output stage and more particularly to an N-channel output stage having a standby mode in which the current injected into the substrate is minimized.
In a field-effect transistor that is turned off, an undesirable current component can be injected into the substrate if a large enough voltage is developed across the device (measured between the drain and the source, i.e. V.sub.DS). In electronic physics, this phenomenon is referred to as channel impact ionization or simply "hot-electron" injection. Generally speaking, the amount of injected substrate current increases rapidly as V.sub.DS is increased. The injected substrate current is undesirable since the voltage of the substrate is locally changed, which in turn may de-bias or otherwise affect the performance of adjacent transistors on an integrated circuit. In addition, on board charge pumps, which maintain a constant substrate voltage, consume excess power through increased cycling necessary to remove the additional charge caused by the injected current.
The problem of injected substrate current for a transistor that is turned off is typically found in an N-channel output stage. The most common form of an N-channel output stage is simply a single N-channel field-effect transistor, wherein the gate receives an input signal, the drain is coupled to the VDD power supply (typically five volts), and the source is coupled to the load. During normal operation the input signal gates current through the output stage to the load. During a standby mode, however, the gate is typically connected to ground to turn the transistor off. In the standby mode, a voltage even greater than the power supply voltage, typically VDD plus a transistor threshold voltage VT, can appear directly across the non-conducting transistor. Although the voltage level at the load is normally at or above ground, it may go below ground due to ringing whenever other circuitry coupled to the load is switched to ground. In the standby mode, therefore, an operating condition can exist wherein the transistor is capable of injecting a significant amount of undesirable current into the substrate of the integrated circuit.
A prior art attempt to minimize the injected substrate current is shown in the schematic diagram of FIG. 1. N-channel output stage 10 includes first and second series-connected field-effect transistors Q1 and Q2. The drain of transistor Q1 is coupled to the power supply VDD at terminal 12, and the source of transistor Q2 is coupled to the output terminal 14. The output terminal 14 is typically coupled to a load impedance. A multiplexer 24 switches the gate of transistor Q2 between an input signal at terminal 16 in a normal operating mode and ground at terminal 18 in a standby operating mode under the command of the standby signal at terminal 22. The addition of transistor Q1 in output stage 10 lowers the voltage across transistor Q2 in the standby mode. Note that the gate and drain of transistor Q1 are coupled together. Therefore the voltage across transistor Q1 is approximately equal to VT, and the voltage across transistor Q2 is lowered by VT. This is true in both the standby and normal operating modes. In the standby mode, leakage current is sufficient to establish the threshold voltage VT across transistor Q1. While the addition of transistor Q1 in output stage 10 reduces the injected substrate current during the standby mode, a significant amount of current is still injected due to the relatively high voltage across transistor Q2.
Therefore, what is desired is an N-channel output stage in which the injected substrate current during the standby mode is minimized to the greatest extent possible.